The invention is directed to a method having the features indicated in the preamble of claim 1. Such a method is known from WO 2010/011838 A1.
Document WO 2004/063560 A1 discloses how a fuel/air mixture can be ignited in a combustion chamber of an internal combustion engine by a corona discharge created in the combustion chamber. For this purpose an ignition electrode is guided through one of the walls, that are at ground potential, of the combustion chamber in an electrically insulated manner and extends into the combustion chamber, preferably opposite a reciprocating piston provided in the combustion chamber. In cooperation with the walls of the combustion chamber that are at ground potential and function as counterelectrode the ignition electrode constitutes a capacitance. The combustion chamber and the contents thereof act as a dielectric. Air or a fuel/air mixture or exhaust gas is located therein, depending on which stroke the piston is engaged in.
The capacitance is a component of an electric oscillating circuit which is excited using a high-frequency voltage created using a transformer having a center tap. The transformer interacts with a switching device which applies a specifiable DC voltage to the two primary windings, in alternation, of the transformer connected by the center tap. The secondary winding of the transformer supplies a series oscillating circuit comprising the capacitance formed by the ignition electrode and the walls of the combustion chamber. The frequency of the alternating voltage which excites the oscillating circuit and is delivered by the transformer is controlled such that it is as close as possible to the resonance frequency of the oscillating circuit. The result is a voltage step-up between the ignition electrode and the walls of the combustion chamber in which the ignition electrode is disposed. The resonance frequency is typically between 30 kilohertz and 3 megahertz, and the alternating voltage reaches values at the ignition electrode of 50 kV to 500 kV, for example.
A corona discharge can therefore be created in the combustion chamber. The corona discharge should not break down into an arc discharge or a spark discharge. Measures are therefore implemented to ensure that the voltage between the ignition electrode and ground remains below the voltage required for a complete breakdown. For this purpose, it is known from WO 2004/063560 A1 to measure the voltage and the current intensity at the input of the transformer and, on the basis thereof, to calculate impedance as the quotient of voltage and current intensity. The impedance calculated in this manner is compared to a fixed setpoint value for the impedance, which is selected such that the corona discharge can be maintained without the occurrence of a complete voltage breakdown.
This method has the disadvantage that the formation of the corona is not optimal and, in particular, an optimal size of the corona is not always attained. Specifically, the corona increases in size the closer the oscillating circuit is operated to the breakdown voltage. To ensure that the breakdown voltage is never reached, the setpoint value of the impedance that must not be exceeded must be so low that a voltage breakdown and, therefore, an arc of a spark, is always prevented. A point that must be considered when specifying the setpoint value of the impedance is that the current-voltage characteristic curve of the circuit driving the transformer is subject to production-related fluctuations. If structural or production-related changes are made to the circuit and the oscillating circuit that cause the current-voltage characteristic curve to change, it may be necessary to redetermine the setpoint value of the impedance using trials, to prevent the situation in which a corona of inadequate size is formed or, in the worst case, a corona is not formed at all.
On the basis of document WO 2010/011838 A1 it is known to control the transformer on the primary side thereof by specifying a setpoint impedance by first determining a so-called baseline impedance at the input of the transformer at a voltage that is so low that a corona discharge does not occur. Starting at a low voltage, the current-voltage characteristic curve at the input of the transformer initially has a linear shape, which indicates that impedance remains the same: The current intensity initially increases in proportion to voltage. The baseline impedance is characteristic for the particular igniter. If a certain voltage is exceeded, the impedance increases, which is indicated by the fact that the intensity of the current measured on the primary side of the transformer is no longer proportional to the voltage, but rather increases at an increasingly slower rate as the voltage continues to increase, until a voltage breakdown occurs between the ignition electrode and one of the walls delimiting the combustion chamber. In the method known from document WO 2010/011838 A1, the setpoint impedance is determined as the sum of the baseline impedance and an additional impedance. The additional impedance is increased in small increments by increasing the voltage until a spark discharge occurs. As soon as a spark discharge is detected, the additional impedance is reduced by an amount that is slightly greater than the preceding increment, in order to prevent further spark discharges and keep the oscillating circuit in resonance. It is therefore possible to hold the current intensity and voltage at the input of the transformer below the level at which a spark discharge can occur, and to limit them to a level at which the corona reaches a maximum size.
The impedance on the primary side of the transformer, at which a corona discharge occurs, and the impedance at which a corona discharge transitions into an unwanted arc discharge or spark discharge can change during the service life of the ignition electrode, which can be disadvantageous for the service life thereof and for the formation of the corona, and can result in non-ideal combustion.